Thermal type flow rate controllers (MFC) and pressure type flow rate controllers (FCS) have commonly been used as a flow rate controllers for various kinds of process gas used in semiconductor manufacturing facilities. Of the two the pressure type flow rate controller has been more widely employed due to its excellent characteristics, such as responsiveness, measuring accuracy, service life, and maintainability, as well as its simplicity in structure.
Flow rate controllers are normally only shipped after flow rate calibrations are performed on each unit using a flow rate reference. Different types of references have been developed for flow rate calibrations. For example, in the Buildup Method/ROR Method, a gas is fed into a chamber and the flow rate of the gas is computed using the pressure of the gas accumulated in the chamber, in the Gas Weight Reduction Method gas accumulated in a chamber is discharged and the flow rate of the gas discharged is computed using the change in weight of the chamber, and similarly, in the Weight Method, gas is fed into a chamber and the flow rate of the gas is computed using the change in weight of the chamber.
Makers of flow rate controllers perform flow rate calibrations on their flow rate controllers by using the flow rate reference chosen at their discretion before the controllers are shipped to the end users. Therefore, once the production facilities choose a particular make of flow rate controller, it becomes impossible to employ a flow rate controller manufactured by another manufacturer when partial renovation or remodeling is conducted. In other words, even if a change is desired, the end user is limited to using the flow rate controller chosen by the manufacturer in the beginning. This makes it difficult to reduce installation costs and the like since each manufacturer has its own flow rate reference for calibrations. The use of the different flow rate references causes inconsistencies in the flow rate values obtained in actual measurements even if the flow rate controllers are manufactured with the same specifications. Therefore, in facilities such as semiconductor manufacturing facilities that require highly accurate flow rate control, it is, extremely difficult to employ flow rate controllers manufactured by different manufacturers at the same time.
As stated above, the buildup method or a weight method is normally used to obtain these flow rate references. For example, as illustrated in FIG. 8, according to the buildup method, first a gas G, having a set flow rate, is supplied into chamber C, having internal volume V, through flow rate controller FM, which performs flow rate calibrations. The relationship between the supply time t and internal pressure P of the chamber is obtained from the readout of a pressure gauge.
Next, as shown in FIG. 9, the pressure rise rate ΔP/Δt of the internal pressure of the chamber is calculated from the measurement results of the supply time t and internal pressure P. Then the flow rate Q (sccm) of the flow rate controller FM is computed using Equation 1 which uses the pressure rise rate ΔP/Δt. Thus, the readout of flow rate controller FM is calibrated based on the computed value.
                    Equation        ⁢                                  ⁢        1                                                                      Q          ⁡                      (                          s              ⁢                                                          ⁢              c              ⁢                                                          ⁢              c              ⁢                                                          ⁢              m                        )                          ⁢                              1            ⁢                          (              atm              )                                            760            ⁢                          (              Torr              )                                      ×        1000        ⁢                              (                          cc              /              l                        )                    ·          60                ⁢                  (                      sec            /            min                    )                ×                                            273              ⁢                              (                k                )                                                                    (                                  273                  +                  T                                )                            ⁢                              (                k                )                                              ·                      V            ⁡                          (              l              )                                      ×                              Δ            ⁢                                                  ⁢                          P              ⁡                              (                Torr                )                                                          Δ            ⁢                                                  ⁢                          t              ⁡                              (                sec                )                                                                        (        1        )            
However, there are problems with the buildup method. For example, the first problem is that accurate measurements of the gas temperature T inside chamber C are difficult. Namely, it has been found that a considerable gap in the measured value exists between the gas temperature T measured inside chamber C and the gas temperature T measured in the vicinity of chamber C. Further, it has been found that when the gas temperature T changes 1° C., the flow rate Q (sccm) varies approximately 0.33% S.P.
The second problem is the low measurement accuracy of the pressure gauge (Baratron), which causes fluctuation in the pressure rise rate ΔP/Δt each time a measurement is taken. In particular, each pressure gauge (Baratron) P is equipped with its own specific temperature characteristics and pressure rise characteristics (linearity). Thus, different pressure rise ranges (span of the pressure rise) and different measuring conditions result in different curves such as that shown in FIG. 9 As a result, the pressure rise rate ΔP/Δt widely fluctuates. Another difficulty is that as the flow rate of the flow rate measuring device becomes larger, a large-sized chamber C is required, and it takes a longer time to take a measurement if the chamber C is made smaller.
The third problem is that the flow rate values (calibrated flow rates) computed with the aforementioned Equation 1 turn out to be different each time actually measured. For example, even when the experiment system, flow rate, measurement time, measuring pressure range, and the like are exactly identical, a highly accurate flow rate calibration can not be expected because in reality the calibrated flow rate value varies only with a change of the flow rate range of the pressure type flow rate controller.
FIG. 10 shows one example of a test result of the flow rate calibration of a flow rate controller based on the buildup method. The figure shows that even when a calibration flow rate is set so that the flow rate error is zero at the flow rate setting of 100%, the error (% F.S.) becomes larger when the flow rate setting value SET (%) is varied.
In addition to flow rate references based on the buildup method, flow rate references based on gas weight have been developed. For example, in the gas weight reduction method, the weight of a gas supply source (a gas cylinder) is measured, and the gas flow rate Q (sccm) is obtained from the amount of weight reduction. In addition, there is a flow rate reference based on the weight method in which the gas flow rate Q (sccm) is obtained from the increased value of the weight of a chamber after gas is fed into the chamber.
However, obtaining a flow rate reference for when the gas weight reduction method or a weight method is employed requires a highly accurate weighing device. Among other problems, this causes the device to become bulky, and a longer time is required for the measurements.
As stated above, there are different types of flow rate references used for the flow rate calibrations of a flow rate controller depending upon the manufacturer of the flow rate controller. In addition, the accuracy of flow rate references during use have been found to be relatively low. This means that the values of flow rate measurements vary each time a different type (maker and model) of flow rate controller is used. For example, in fields such as semiconductor manufacturing, and the like for which finer processing is required, slight differences (errors) in the flow rate calibrations of a flow rate controller can cause a detrimental effect on the entire processing.
In semiconductor manufacturing facilities and the like, corrosive gases are widely used as a process gas, often causing errors with the flow rate controller due to the corrosiveness. This problem is detrimental not only to processing with a flow rate controller but also to the reference flow rate controller (master controller) used with the pressure control type flow rate reference. Therefore, when the gas used for the flow rate calibration is corrosive, most likely, the shape of the orifice of the aforementioned master controller becomes deformed, the dimensions of the internal diameter of the master become varied, and the orifice becomes clogged with the corrosive products. Thus the function of a master controller is ultimately lost.    Patent Document 1: Japanese Patent No. 3580645    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-63265    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-137528